1. Field of the Invention
The present invention relates to a III-V group compound semiconductor light emitting device such as a semiconductor laser device or a light emitting diode (LED) device, and a method for producing the same.
2. Description of the Related Art
A III-V group compound semiconductor light emitting device such as a semiconductor laser device or an LED device generally has at least one (typically a plurality of) crystal growth layer of the p-type conductivity (hereinafter, referred to also as the "p-type layer") and at least one (typically a plurality of) crystal growth layer of the n-type conductivity (hereinafter, referred to also as the "n-type layer"). These crystal growth layers are typically formed by using a crystal growth method such as a liquid phase epitaxial (LPE) method, a molecular beam epitaxial (MBE) method, or a metalorganic chemical vapor deposition (MOCVD) method, which is excellent in mass productivity and allows a very thin film to be grown.
In the LPE method, although a high quality semiconductor crystal can be formed with a relatively simple apparatus, it is difficult to produce a uniform crystal over a large area. On the other hand, the MBE method and the MOCVD method are more suitable processes for mass production, and are widely used at present. The MBE method is a process in which solid elements forming a compound semiconductor are heated in a high vacuum and a substrate is irradiated with beams of the evaporated elements. With this method, a pure crystal can be relatively easily obtained. In the MOCVD method, under an atmospheric pressure or a pressure depressurized to about 1/10 atm, elements forming a compound semiconductor are carried in a gaseous form such as an organic compound or a hydrogen compound, so as to be chemically reacted on a substrate, thereby forming an intended compound semiconductor.
FIG. 1 is a cross-sectional view illustrating a typical structure of a III-V group compound semiconductor laser device produced by using the MOCVD method.
In the cross-sectional view of FIG. 1, an n-type GaAs buffer layer 1, an n-type AlGaAs cladding layer 2, an AlGaAs active layer 3, a p-type AlGaAs first cladding layer 4, a p-type GaAs etching stop layer 5, a p-type AlGaAs second cladding layer 6 and a p-type GaAs protective layer 7 are deposited in this order on an n-type GaAs substrate 14, thereby forming a layered structure. In this layered structure, the layers above the p-type GaAs etching stop layer 5 form a stripe-shaped mesa structure (a mesa stripe). An n-type AlGaAs current blocking layer 8, an n-type GaAs current blocking layer 9 and a p-type GaAs planarizing layer 10 are buried on both sides of the mesa stripe.
Moreover, a p-type GaAs contact layer 11 is formed on the p-type GaAs protective layer 7 and p-type GaAs planarizing layer 10. A p-side metal electrode 12 and an n-side metal electrode 13 are respectively formed on the p-type GaAs contact layer 11 and on the reverse surface of the n-type GaAs substrate 14 by, for example, a vapor deposition method.
FIG. 2 is a cross-sectional view illustrating another typical structure of a III-V group compound semiconductor laser device produced by using the MOCVD method.
In the cross-sectional view of FIG. 2, a Se-doped n-type GaAs buffer layer 22, a Se-doped n-type AlGaAs cladding layer 23, an undoped AlGaAs active layer 24, a Zn-doped p-type first cladding layer 25 and a Se-doped n-type AlGaAs current blocking layer 26 are formed in this order on an n-type GaAs substrate 21. A portion of the n-type current blocking layer 26 is removed in a stripe-shaped pattern, thereby forming a current path 17. A Zn-doped p-type second cladding layer 28 and a Zn-doped p-type contact layer 29 are formed on the n-type current blocking layer 26 including the stripe-shaped portion 17. A p-side electrode 18 and an n-side electrode 19 are respectively formed on the p-type contact layer 29 and on the reverse surface of the n-type GaAs substrate 21.
FIG. 3 is a diagram schematically illustrating a structure of a vapor deposition apparatus of a depressurized horizontal RF heating furnace type which can be used for growing the semiconductor layers included in the semiconductor laser device illustrated in FIG. 1 or 2.
In the apparatus illustrated in FIG. 3, trimethylgallium (TMGa), trimethylaluminum (TMAl) or trimethylindium (TMIn) is used as a III-group material compound; arsine (AsH.sub.3) or phosphine (PH.sub.3) as a V-group material compound; monosilane (SiH.sub.4), disilane (Si.sub.2 H.sub.6) or hydrogen selenide (H.sub.2 Se) as an n-type dopant material; and diethylzinc (DEZn), dimethylzinc (DMZn) or trimethylarsenic (TMAs) as a p-type dopant material. Carbon tetrachloride (CCl.sub.4) may also be used as a carbon source.
In a crystal growth process, a substrate is placed inside a reaction chamber (growth chamber) 30, the internal pressure of the reaction chamber 30 is set to a predetermined value (e.g., about 76 Torr), and the substrate temperature is set to a predetermined value (e.g., about 700.degree. C.) using an RF coil 31. Then, mass flow controllers (MFCs) and valves are appropriately controlled to appropriately select, and set the flow rate of, the respective materials from material sources 32 to 38 and hydrogen supplied from a hydrogen source through a line 39 so as to supply them into the reaction chamber 30 through respective supply lines 40 to 43, thereby growing an intended semiconductor layer on the substrate. Any unwanted gas which may exist in the reaction chamber 30 is exhausted through a line 44.
For example, when forming an n-type AlGaAs layer, AsH.sub.3, TMGa, TMAl and an appropriate n-type dopant material are supplied onto the substrate. When forming a Zn (zinc) doped p-type AlGaAs layer, AsH.sub.3, TMGa, TMAl and DMZn or DEZn are supplied onto the substrate. When forming a C (carbon) doped p-type AlGaAs layer, TMAs, AsH.sub.3. TMGa and TMAl are supplied onto the substrate.
In the MOCVD method, it is likely that organic matter or hydrogen generated after a chemical reaction may be introduced as an impurity into a growing compound semiconductor layer. Particularly, carbon (C) contained in organic matter, when introduced into the compound semiconductor layer, may act as a p-type dopant. Thus, it is likely that a certain amount of carbon may be present in the grown compound semiconductor layer even when carbon tetrachloride (CCl.sub.4) is not supplied.
For example, when forming a p-type AlGaAs layer with a high concentration of zinc added thereto, the crystal growth process is performed while reducing the substrate temperature from about 700.degree. C. to about 600.degree. C. during the growth process. Then, a certain amount of carbon atoms are introduced into an AlGaAs crystal layer obtained through the crystal growth process by the MOCVD method. As the Al mole fraction increases carbon atoms are increasingly likely to be introduced. Therefore, carbon atoms, though at a concentration lower than the predetermined zinc atom concentration, will be present in the grown p-type AlGaAs layer.
When forming a p-type AlGaAs layer with a high concentration of carbon added thereto, the crystal growth process is performed while reducing the supply ratio between the V-group material and the III-group material (V/III ratio) from about 60 to about 2 during the growth process. Also in this case, carbon atoms will be present at a concentration of, for example, about 1.times.10.sup.17 cm.sup.-3 in the adjacent n-type AlGaAs layer to which carbon is not intended to be added.
Moreover, as can be seen from the above description of the structure of the apparatus illustrated in FIG. 3, in the formation of a compound semiconductor layer by the MOCVD method, Se or Si is used as the n-type dopant element, and Zn, Mg, C, or the like, is typically used as the p-type dopant element. These dopants should be controlled so as to be present at a predetermined concentration in the intended crystal growth layer. When one of the dopants is used alone, the doping conditions such as the concentration may be controlled relatively easily. However, these dopants diffuse as the substrate is heated during the growth process, whereby an intended concentration profile may not be obtained. Moreover, in the case of a layered structure, complex mutual diffusion of the dopants may occur depending upon the combination of the materials of the adjacent layers or the combination of the dopant types used.
For example, Zn, Be, Mg, or the like, which are II-group elements and used as p-type dopants, are impurities which may diffuse relatively easily, and are difficult to control. Particularly, when such an element is added at a high concentration, precipitation at the interface between the p-type crystal growth layer and the n-type crystal growth layer (the p-n interface), or diffusion toward the substrate surface, occurs significantly.
On the other hand, carbon, which also acts as a p-type dopant, has a small diffusion coefficient, and thus is often used in HBTs or HEMTs. However, when carbon is used as the p-type dopant in a semiconductor light emitting device, although the carbon itself does not diffuse, diffusion of Si or Se used as an n-type dopant may cause a problem that the C doped p-type layer is inverted to the n-type.
Moreover, when only carbon as a p-type dopant is added into a crystal layer of a III-V group compound semiconductor, in order to realize an intended amount of addition, it is typically required to set the supply ratio between the V-group material and the III-group material (V/III ratio) at a small value. However, under such a setting, it is likely that an impurity such as oxygen or water, in addition to carbon, may be introduced into the crystal layer, whereby the electrical and/or optical characteristics of the obtained p-type III-V group compound semiconductor crystal layer may deteriorate.
On the other hand, when only zinc as a p-type dopant is added into a crystal layer of a III-V group compound semiconductor, in order to realize an intended amount of addition, it is typically required to set the substrate temperature during the crystal growth process at a small value, as described previously. However, under such setting, it is likely that an impurity such as oxygen or water is introduced into the crystal layer, whereby the electrical and/or optical characteristics of the obtained p-type III-V group compound semiconductor crystal layer may deteriorate.
Furthermore, due to the mutual diffusion of Zn as a p-type dopant and Se as an n-type dopant, the Zn-doped layer, which should be of the p-type, may not always be of the p-type; i.e., conductivity type inversion may occur. For example, in the structure illustrated in FIG. 2, the p-type first cladding layer 25 may be inverted to the n-type, whereby the n-type cladding layer 23 and the n-type current blocking layer 26 may be short-circuited with each other. Alternatively, a portion of the n-type cladding layer 23 directly beneath the stripe-shaped portion 17, in the vicinity of the active layer 24, may be inverted to the p-type, whereby an increase in the operating current (a decrease in reliability) may occur.
In order to suppress such mutual diffusion, Si may be used instead of Se as an n-type dopant. Si is a dopant which is typically less likely to diffuse and has an effect of suppressing diffusion of a p-type dopant at the p-n interface into the n-type layer. However, such an effect of suppressing the p-type dopant diffusion into the n-type layer may contrarily lower the operating characteristics of the produced light emitting device.
In particular, when the n-type dopant in the structure illustrated in FIG. 2 is Se exhibiting no diffusion suppressing effect, due to the diffusion of Zn as a p-type dopant added to the p-type cladding layer 25, the carrier concentration of the p-type cladding layer 25 in regions beneath the current blocking layer 26 becomes lower than that in another region beneath the stripe-shaped portion 17. On the other hand, when Si is used in stead of Se as an n-type dopant, Si remains in the n-type cladding layer 23 and the n-type current blocking layer 26, and Zn as a p-type dopant added to the p-type cladding layer 25, except for a small amount thereof diffusing into the active layer 24, remains in the p-type cladding layer 25 due to the diffusion suppressing effect of Si. As a result, the carrier concentration of the p-type cladding layer 25 is substantially the same in the region beneath the stripe-shaped portion 17 and in the region beneath the current blocking layer 26. Therefore, the resistance of the p-type cladding layer 25 decreases while the leakage current increase, whereby there occurs a problem of a large operating current.
As described above, in accordance with the conventional techniques, it is not possible to reliably suppress diffusion of dopants contained in the respective compound semiconductor layers of the III-V group compound semiconductor light emitting device. Therefore, it is difficult to reliably control the dopant concentration or the conductivity type of the respective layers, whereby the operating characteristics of the resultant light emitting device may deteriorate.